In the fabrication of micro circuits a photoresist coated silicon wafer is exposed to ultra-violet light through a mask containing the circuit patterns to be formed on the wafer. The photoresist is developed or otherwise partially removed leaving the circuit pattern outlined on the wafer. During actual buildup of a complete circuit on the wafer, the wafer may have to be repeatedly coated with the photoresist material. The mask and, therefore, the wafer has a plurality of identical circuits arranged in rows and columns and once the process is completed the wafer is cut into rows and columns to form individual circuits.
A critical part of the foregoing described process is in the provision of silicon wafers coated with the light sensitive photoresist material. As the state of the art in the fabrication of micro circuits advances, it becomes more and more critical that the silicon wafers have a film of photoresist deposited thereon which is highly uniform and lacking defects such as striations.
The technique most widely in use for the coating of silicon wafers with photoresist is known as spinning. This method entails placing a drop of liquid photoresist material on a wafer which is spun. Due to the aerodynamic and centrifugal forces the photoresist eventually covers the entire wafer in a thin film. This method has the disadvantage of poor uniformity in coating thickness. For example, spin coating causes intrinsic and induced striations which are grooves or channels formed in the surface of the coating.
Intrinsic striations are caused by the nature of spin coating itself due to the outward radial movement of the liquid photoresist.
Induced striations are due to specks such as dust or foreign matter in the photoresist or on the wafer, as well as previously etched patterns on a wafer.
The spin coating of a wafer already etched with an array of patterns also exhibits nonuniformities due to the piling-up and thinning-out of photoresist in different areas of the etched pattern depending upon its location and orientation relative to the center of the wafer.
These striations and nonuniformities, which are areas of differing thickness of the photoresist layer, result in varying degrees of exposure sensitivity which lead to unreliable photoresist development such as removing too little or too much photoresist during development. This ultimately results in unreliable or failed circuit elements in the finished microcircuits.
Another disadvantage of the spin method of coating is that it is wasteful of the photoresist, with losses typically exceeding 99% of the material.
The present invention relates to an apparatus for coating wafers with photoresist material which overcomes the foregoing disadvantages and provides more economical coatings of uniform thickness free of the striations and nonuniformities encountered in spin coating.